I personally recommend Noodler's Bulletproof Black. It writes consistently and reliably on any paper I've tried, it's waterproof, and it's reasonably priced. If you're interested in fountain pens as a hobby rather than as practical writing implements you might have different recommendations.
I did have one mold issue with Private Reserve in my TWSBI, and to be safe I have avoided using it in my Vanishing Point. I may be being overcautious though; I'm not sure how much to extrapolate from that one incident. My mother is an artist and has used several dozen Private Reserve inks spanning over a decade, all without any issues.
I'm 2 years in and I think I'd just designate a pen to be dedicated to the known Noodler's "problematic" colors(BSB and Mass 54th). I just cleaned out 4 pens, some of which had dried ink in them, all of them were fine.
I'm partial to Noodler's Heart of Darkness and X-Feather.
Ditto on Bay State Blue though. Beautiful ink, but notorious for ruining pens in various ways.
A good resource for fountain pen lovers is https://www.nibs.com/
They do repair and custom work too.
https://www.jetpens.com/blog/great-beginner-fountain-pens-th...
(ed: I’ve had magnificent $3 chinese pens, awful $3 chinese pens, and some that can be improved with fiddling. Most Americans prefer broader, more “buttery” writing, which can be hard to get from the average Chinese nib. For most beginners, then, I’d say get a basic medium nib pen from an American/European/Japanese brand.)
Refillable pens can be had for not a whole lot more, but the ink reservoir will be smaller, so, even though they usually come with a free cartridge or two, you may end up wanting to buy a few extra cartridges or a bottle of ink in order to get enough writing time to really get a feel for the pen.
On the slightly cheaper side, the Pilot MR is usually a bit under $20. It's more of a traditional fountain pen look. Pilot Metropolitan is a common name for it (or more specifically a type or MR, just different color/pattern options IIRC).
Then there's all the Chinese pens, some of these are fine but it's hit or miss. I'd suggest sticking with a known manufacturer for a first pen since they have better quality control (especially on the nibs).
Be wary of buying pens on Amazon, their counterfeiting problem extends to fountain pens. Pen retailers like JetPens or Goulet Pen Company are safer since you know they're coming through the real distribution channel.
http://www.deskoflori.com/blog/2015/4/16/my-experience-with-...